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Acids and Bases Acid-Base Definitions –Arrhenius definition is limited to aqueous solutions, and only allows for one kind of base, those with hydroxide ions. –Bronsted-Lowry is a more general definitions, water can now act as a base. –Lewis - the most general definition

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Acids and Bases Terms to know: –proton - H + ion –hydronium ion - H 3 O + ; results from water reacting with H + H 2 O + H + --> H 3 O + –conjugate base - whatever is left from an acid after a proton has been donated –conjugate acid - whatever has been formed when a proton has been accepted by a base

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Acids and Bases Acid Strength –defined by the equilibrium position of the dissociation reaction –Strong Acid - equilibrium lies far to the right, i.e. the acid is 100% ionized –the conjugate base of a strong acid is a much weaker base than water, i.e. the conjugate base will not accept the H + –the stronger the acid the weaker the conjugate base

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Acids and Bases Weak acid –not 100% ionized –equilibrium lies to the left –very little HA is ionized –the conjugate base of a weak acid is a stronger base than water, the conjugate base is more likely to accept an H + than water –the weaker the acid, the stronger the conjugate base

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Acids and Bases pH scale –Because the [H + ] in any solution is generally quite small, it is easier to use the pH scale to represent a solution’s acidity. –pH comes from the Danish…potenz or strength of the H + ion –pH = - log[H + ] –pOH = - log [OH - ]

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Acids and Bases pH is a log scale –when the pH changes by one, the [H + ] concentration changes by a power of 10. A solution with a pH of 3 has 10 times more H + than a solution with a pH of 4, and 100 times more H + than a solution with a pH of 5. –As pH decreases, the [H + ] increases. –Rule for significant figures for logarithms - the number of places after the decimal point is equal to the number of significant figures in the original number pH = - log 1.0 x M (2 significant figures in 1.0 x ) pH = 9.00 ( 2 places after the decimal point for significant figures)

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Acids and Bases Acid-Base Equilibria…or doing acid-base problems –the aqueous solutions contain many components –you must be able to determine which components are most significant and which can be ignored –you must be able to determine which reaction is most important of all the possible reactions

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Acids and Bases Ex: Calculate the pH of 1.0 M HCl –First: Determine the major species HCl is a strong acid, 100% ionized Major species then will be H +, Cl - and H 2 O –Since this is an acidic solution with “lots” of H +, the [OH - ] will be insignificant. –What about H 2 O H + + OH - ? Will any H + come from this reaction? –Le Chatelier’s principle tells us that the reverse reaction will be favored because of the high concentration of H + from the HCl, so we can ignore this reaction as a source of H +. Thus the pH = - log (1.0) = 0.00

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Acids and Bases pH of Weak Acid Solutions Calculate the pH of 1.0 M HF –Determine the major species: HF (because it is a weak acid, very little of it ionizes), and H 2 O –Which of these major species will provide the H + ions? Consider the K a and K w

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Acids and Bases HF H + + F - K a = 7.2 x H 2 O H + + OH - K w = 1.0 x Because the K a of HF is greater than the K w, HF is a stronger acid than H 2 O, and will be the primary contributor of H + in solution. Use the K a for HF to determine the [H + ] at equilibrium, and then determine the pH.

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Acids and Bases Simplifying acid-base equilibrium calculations –If K a is small, then “x” is small relative to the original concentrations. – x becomes essentially 1.00 in these calculations –Is this a valid assumption? Please check at the end! –Compare x to [HA o ], if x < 5% of [HA o ], then the assumption is valid

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Acids and Bases The pH of a mixture of weak acids –Calculate the pH of a solution that contains 1.00 M HCN (K a = 6.2 x ), and 5.00 M HNO 2 (K a = 4.0 x ). –Determine the major species: HCN, HNO 2, and H 2 O –Compare K a ’s and K w. The K a for HNO 2 is larger than K w and the K a for HCN HNO 2 H + + NO 2 - is the reaction of interest.

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Acids and Bases Acid-Base Properties of Salts Salt = an ionic compound –salts dissolve in water to form ions –sometimes these ions can react with water to form weak acids or weak bases

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Acids and Bases Neutral Salts When these salts dissolve in water, the pH does not change. There is no reaction of the ions from the salt with water. The anion and cation are derived from a strong acid and a strong base, respectively.

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Acids and Bases Ex. NaCl is a neutral salt. Na +, the cation, can be considered as coming from NaOH, a strong base. Cl -,, the anion, can be considered as coming from HCl, a strong acid. Remember the conjugates of strong acids and strong bases are very weak, weaker than water, and so are unlikely to react with water to reform the acid or base.

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Acids and Bases Acidic Salts –The cation from an acidic salt will react with water to form a weak base and H +. –The cation must have come from a weak base. –Ex: NH 4 Cl. The NH 4 + comes from a weak base, and will react with water to re-form the weak base and H + (H 3 O + ). The Cl - comes from a strong acid, and cannot react with water to re- form HCl. NH H 2 O NH 3 + H 3 O +

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Acids and Bases Salts that are derived from both a weak acid and a weak base –We can predict whether the solution will be acidic or basic based on the K a of the acidic ion and the K b of the basic ion. –K a > K b pH < 7, acidic solution –K b > K a pH > 7, basic solution –K b = K a pH = 7, neutral solution

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Acids and Bases The effect of structure on acid-base properties –Any molecule containing hydrogen could theoretically act as an acid. However, most of these molecules don’t. –Organic molecules with lots of C-H bonds are not acidic because the C-H is both strong, and relatively nonpolar, so there is no tendency to lose H + –However, while the H-Cl bond is stronger than the C-H bond, the H-Cl bond is very polar, and there is a strong tendency to lose H +.

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Acids and Bases Two Factors that determine the acidity of a molecule containing X-H –strength of the X-H bond –polarity of the X-H bond Consider polarity of the X-H bond –H-F > H-Cl > HBr > HI in terms of polarity –H-F > H-Cl > H-Br > HI in terms of bond strength HF is a weak acid, HCl, HBr, and HI are strong acids even though the H-F bond is very polar, the H-F bond is very strong, so HF is a weak acid

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Acids and Bases Oxygen is very electrognegative. With increasing number of oxygen atoms, the oxygen atom draw more and more of the electron density towards themselves, thus polarizing and weakening the O-H bond,

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Acids and Bases This behavior also occurs with hydrated metal ions. Al +3 is strongly attracted to water, weakening the O-H bond. The greater the charge on the metal ion, the stronger the acidity.

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Acids and Bases The greater the electronegativity of X in the H-O-X grouping, the stronger the acid. –X is able to withdraw electron density from the H-O bond, thus weakening and polarizing the H-O bond, resulting in a stronger acid

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Acids and Bases Comparison of Electronegativity of X and K a

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Acids and Bases Acid-Base Properties of Oxides An H-O-X grouping in a molecule may result in a molecule that behaves as a base or an acid. The nature of the O-X bond will determine the acidic or basic behavior of the molecule

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Acids and Bases If X is very electronegative, the O - X bond will be covalent (not as polar because of similar E.N.’s), and strong. The O - X bond will remain intact in water, while the polar and weak H - O bond will break, resulting in acidic behavior. –e.g. HClO 2 or H 2 SO 4 If X is not very electronegative, the O - X bond will be ionic, and can be broken in water, resulting in basic behavior. –e.g. NaOH or KOH